The present invention relates to drill entry or back-up materials for drilling small holes in substrates and more particularly to improved drill back-up materials for use in small bore drilling of circuit boards.
In recent years, there has been an accelerating trend towards simultaneous circuit size reduction and expansion of function in high performance electronic systems. Printing wiring board (PWB) technology has played an integral role in this evolution. Surface mount technology (SMT) has introduced new requirements in the design of PWBs, such as the use of substrates with fine lines and close spacing, small diameter plated through holes (PTH) and vias, and chip carriers with large numbers of fillet solder joints.
In multi-layer printed wiring board construction (MLPWB), the evolution of PWB laminates (often composed of multiple thin laminates or "thin lams") has been prompted by greater device complexity, as well as the need to package these devices in a smaller volume. Maximum circuit integrity under adverse conditions also is a design criterion. Selection of materials which exhibit desirable characteristics is one method for improving PWB performance. Both the resin system or matrix as well as the reinforcing materials need to be tailored, as does their combination, in order for PWBs to continue to keep pace with chip development. Resin systems, for example, generally are found to require a compromise from among properties such as low bulk coefficient of thermal expansion (CTE), low dielectric constant (E.sub.r), high thermal stability, high glass transition temperature (T.sub.g), and ease of processing.
The foregoing requirements, perhaps, can be better appreciated when it is realized that conventional (and future) ceramic chip carrier materials require an in-plane CTE as low as 0 ppm/.degree. C. and as high as 15 ppm/.degree. C. (ppm, for present purposes, representing 10.sup.-6). In-plane CTE values in this range also will facilitate future implementation of direct die-to-bond attachment. A close Z-axial composite expansion to match copper (17 ppm/.degree. C.) also is important to reduce stress in PTHs. Reduced dielectric constant (say, under 3.0 at 1 KHz-10 MHz) becomes increasingly desirable at high device clock rates since this minimizes propagation delay. Additional problems requiring addressing include LCCC (leadless ceramic chip carrier), solder joint thermal fatigue and plated hole barrel cracking.
MLPWBs typically are constructed from a series of individual PWBs having insulative layers separating each PWB. Standard multi-layer boards, for example, can range from about 2 to 20 layers, each laminant ranging in dielectric thickness from about 1 to 250 mils. Heretofore, it has been possible to successfully drill holes (PTHs and vias, for example), down to about 0.006 inch in diameter or less in PWBs constructed of conventional material. However, more recently, MLPWBs have resorted to a variety of exotic materials in order to meet the increasing demands made upon them. It will be appreciated that the pad on each layer of the MLPWB has to have the hole precisely centered therethrough, especially as chip density increases. It will be appreciated that some manufacturers require an accuracy of about 0.0002 inch. Wandering of the drill can result in an off-centered hole through the pad, thus compromising the efficacy of the board, or can result in breakage of the drill.
Consequently, the art has resorted to materials in sheet form which are placed on both sides of the board to be drilled, the upper material serving as a drill entry sheet, while the lower sheet serving as a back-up sheet. For present purposes, both sheets are described as "back-up" sheets often herein. The drill entry sheet acts as a bushing for the drilling operation, while the back-up or lower sheet serves as a run out function so that the full diameter of the drill can penetrate the lowestmost board, if MLPWBs are being drilled. One back-up material for this use is disclosed in U.S. Pat. No. 4,643,963. Nevertheless, improvements in back-up material composition is an ever increasing need in the art.